Radiation therapy for the treatment of cancer is well known. Typically, radiation therapy involves focusing a beam of X-ray radiation into a target volume to diagnose an afflicted area or monitor a tumor or lesion. A beam of high energy x-ray radiation or electron radiation (“therapeutic radiation”) is subsequently directed into the monitored area to treat the area. The area is further monitored to ensure an appropriate positioning of the therapeutic radiation beam. Radiation therapy devices are commonly equipped with linear accelerators used to generate either (or both) electron radiation or X-ray radiation.
A typical configuration for a radiation therapy device includes a radiation source for emitting the therapeutic x-ray radiation, an X-ray source for emitting the X-ray radiation for imaging and one or more imaging devices corresponding to a radiation source for receiving incoming radiation after passing through the target volume. The beams collected by the imagers are used to generate a display (i.e., one or more images) of the targeted volume.
Newer technology and advanced techniques allow for improved image collection. A recent development in the field of computerized tomography is the use of a cone-beam computerized tomography system. A cone beam computerized tomography system is similar to that of a conventional computerized tomography system, with the exception that an entire volumetric image is acquired through rotation of the source and imager. This is made possible by the use of a two-dimensional (2D) imager. A plurality of two dimensional images at various angles is collected. From these two dimensional images, the cone beam computerized tomography system reconstructs three-dimensional images according to various methods and algorithms.
In conventional computerized tomography systems, one or more 2D slices are reconstructed from one dimensional projections of the patient, and these slices may be combined to form a three dimensional (3D) image of the patient. In contrast, in cone beam computerized tomography, a fully 3D image is reconstructed from a plurality of 2D projections. Cone beam computerized tomography offers a number of advantages, including: formation of a 3D image of the patient from a single rotation about the patient (whereas conventional computerized tomography typically requires a rotation for each slice); spatial resolution that is largely isotropic (whereas in conventional computerized tomography the spatial resolution in the longitudinal direction is typically limited by slice thickness); and considerable flexibility in the imaging geometry.
Radiation therapy subjects typically receive treatment in a supine position. Traditional configurations consist of an overhead radiation source directing radiation into a targeted volume within a prone subject positioned directly below the radiation source. Unfortunately, diagnostic imaging performed during the application of radiation therapy to a target volume from a single angle and direction may often be ineffective. For example, diagnostic imaging from conventional computerized tomography systems may be obscured by other content (e.g., anatomy) within the target volume.
In recently developed radiation therapy devices, the structures supporting the radiation source have become configured to be capable of rotation along one axis (typically, around the target subject). Even more recently, radiation therapy devices with a plurality of supporting structures each attached to a radiation source or imager configured to rotate around the same axis have been developed. FIG. 1 depicts a conventional radiation therapy device with two rotatable gantries, each supporting a radiation source. FIG. 1 includes a first gantry attached to a therapeutic radiation source, a second gantry attached to a diagnostic radiation source, a multiple-energy imaging unit for receiving radiation passing from both radiation sources, and a couch upon which the patient subject is positioned. The gantries and affixed radiation sources rotate around the subject to generate a volumetric image.
However, the solution of rotating one or more support structures around a single axis for diagnostic purposes restricts the volumetric imaging acquisition process to very limited trajectory shapes. For example, a support structure which rotates around a shared axis is capable of only circular, spiral and spherical trajectories. Unfortunately, volumetric imaging along a single axis of rotation may be incomplete. Furthermore, rotatable gantries typically rotate along a fixed circumference and may not be adjustable. Accordingly, this prevents images from being acquired at a greater proximity (“zooming”) which would allow for a greater resolution of a smaller target volume.